This invention relates to improved appliance doors and panels, and more particularly to an improved method of manufacturing such doors and panels. Even more particularly this invention relates to a two-step manufacturing procedure for producing a blow molded plastic refrigerator door assembly that is specially designed to reduce the impact thereon of thermal bow.
As used hereinafter, thermal bow refers to that characteristic common to most of the plastic articles referred to herein--i.e., the tendency of plastic to move away from cold and toward heat. A refrigerator/freezer presents an extreme temperature difference of 60.degree. to 90.degree. F. from inside to outside, and within approximately two inches of distance. In a horizontal plane, where gravity is a contributor, a minimal temperature deviant will exhibit the bow condition. Previous attempts to create a plastic solution have not succeeded in managing the thermal bow characteristics. In normal conditions a plastic refrigerator door would distort outwardly, overcoming formed structural details intended to stiffen the part. Several features integrated into the new design methodology described below are believed to manage the thermal bow condition.
The conventional method of constructing refrigerator doors includes providing a stamped and formed metal shell (exterior door panel) which is adjoined to a vacuum formed plastic shell (interior door panel), either by enclosing the perimeter of the plastic shell with formed flanges on the metal shell, or by a plurality of retaining strips and screws. A gasket may also be attached to the interior shell by means of the same retaining strips and screws. Then, according to the former, a foaming agent that solidifies to become a rigid insulating material may be blown in-between the interior and exterior shells, or according to the latter, a rigid foam insulation may be inserted therebetween prior to their connection to provide additional structural support. Finally, the door handle and other decorative features are attached to the exterior shell, and pivot pin bushings are inserted into the four corners on the top and bottom ends of the exterior shell to allow reversible hanging of the door on a refrigerator. The metal shell, due to its rigidity and conductivity, does not visually exhibit the characteristic of thermal bow. There are, however, several problems using conventional refrigerator door design details for a plastics solution such as, first, the lack of structural and design features fully effectively to manage the thermal effects resulting from the internal/external temperature difference; second, the use of rigid insulation results in a direct transfer of thermal effects from the interior surface to the exterior surface; and third, the number of steps required to assemble the door.
Extrusion blow molding using special plastic resins, such as Acrylonitrile Butadiene Styrene (ABS), Polycarbonate (PC), Polyphenylene Oxide (PPO) and Polyvinyl Chloride (PVC), is a manufacturing process which is characterized by the extrusion of a tubular, molten plastic parison between opposing mold plates prior to their closure. Once the mold plates are closed the force of air introduced by a blow pin into the center of the captured parison causes the molten plastic to expand and to conform to the mold cavities and to any incongruities or inserts projecting from the confronting internal faces thereof. Efforts to use a one-step blow molding process for refrigerator or freezer door manufacture have previously been unsuccessful due to the failure of such doors adequately to manage the above-noted thermal bow problems, and material distribution problems caused by disproportionate blow ratio conditions on opposing sides of the pinch line.
It is therefore an object of this invention to provide a two-step manufacturing process and design details for the construction of an appliance door, the structural integrity of which resists the effects of thermal bow.
Another object of this invention is to provide an improved, hollow refrigerator or freezer door containing insulation types alternative to rigid foam, which enhance structural considerations and thermal bow management of such doors.
Another object of this invention is to provide a refrigerator door, the construction of which utilizes more efficient manufacturing procedures and allows for improved quality, thereby rendering such refrigerator doors more economical to produce.
Another object of this invention is to illustrate that a simplified mold design for appliance doors is possible without the need for inclusion of sophisticated slide or cam action, thereby reducing initial and on-going production costs.
Yet another object of this invention is to provide a novel refrigerator door and manufacturing method therefor which permits product line distinction, design options and model changes to be effected on a selective basis without the need for total retooling.
Other objects of this invention will become apparent hereinafter from the specification and the recital of the appended claims, particularly when considered in conjunction with the accompanying drawings.